FIG. 5 is a circuit diagram that illustrates a known control system for a vehicular generator.
In FIG. 5, the known control system for a vehicular generator includes a control circuit 1 installed on a vehicle for adjusting the voltage of power generated, a vehicle-mounted generator 2 having a field coil 201, an armature coil 202 and a rectifier 203, a key switch 3 that is turned on when the vehicle is driven to operate, and a vehicle-mounted battery 4 adapted to be charged by the power generation voltage output from the armature coil 202 through the rectifier 203.
The control circuit 1 has a voltage detection circuit for detecting the terminal voltage of the battery 4 (hereinafter referred to as “the battery voltage”), and adjusts the generated voltage to a predetermined value by controlling to turn a field current supplied to the field coil 201 on and off in accordance with the battery voltage.
For this purpose, the control circuit 1 includes an output terminal B hereinafter referred to as “B terminal”) connected to the battery 4, an input terminal S (hereinafter referred to as “S terminal”) for detecting the battery voltage, and a power supply input terminal IG (hereinafter referred to as “IG terminal”) connected to the battery 4 through the key switch 3.
In addition, the control circuit 1 includes a power MOSFET 101 connected to the field coil 201, a blocking (reverse current prevention) diode 102 connected to an output terminal of the power MOSFET 101, a transistor 104 for turning the power MOSFET 101 on and off, resistors 103, 105 connected to the IG terminal, a comparator 106 for turning the transistor 104 on and off, a resistor 107 and a variable resistor 108 both connected to the S terminal (external voltage detection terminal), and a resistor 109 and a Zener diode 110 connected to the IG terminal.
A junction between the resistors 103, 105 are connected to an output terminal of the transistor 104 and a gate terminal of the power MOSFET 101.
The comparator 106, the resistor 107 and the variable resistor 108 together constitute a voltage detection circuit for detecting the battery voltage.
That is, the resistor 107 and the variable resistor 108 serve to divide the battery voltage to generate a detection voltage, which is input to the comparator 106.
A junction between the resistor 107 and the variable resistor 108 is connected to a comparison input terminal (+) of the comparator 106 which has a reference input terminal (−) impressed with a reference voltage Vref.
In FIG. 5, when the key switch 3 is turned on (closed) upon starting of the vehicle, a gate voltage of the power MOSFET 101 becomes a voltage value equal to the battery voltage divided by a voltage division ratio of the resistors 103, 105, whereby the power MOSFET 101 is made into a conductive state. As a result, a field current is supplied to the field coil 201 so that the generator 2 becomes able to generate electric power.
On the other hand, the Zener diode 110, to which the battery voltage is supplied through the resistor 109, constitutes a constant-voltage power supply V1 that generates a constant voltage based on the battery voltage. Also, a reference voltage Vref (a reference of comparison to the battery voltage) in the comparator 106 is generated based on the constant-voltage power supply V1.
When the generator 2 starts generating electricity in accordance with the engine starting of the vehicle, the voltage detection circuit 107, 108 in the control circuit 1 detects the battery voltage from the S terminal, and inputs it to the comparison input terminal (+) of the comparator 106.
When this detected voltage becomes higher than the predetermined voltage Vref set at the reference input terminal (−), the transistor 104 becomes conductive due to an ON output of the comparator 106, thereby interrupting or turning off the power MOSFET 101. As a result, the field current is decreased to reduce the power generation voltage of the generator 2.
On the other hand, when the detected value of the battery voltage falls below the reference voltage Vref, the transistor 104 is interrupted or turned off due to an OFF output of the comparator 106, so that the power MOSFET 101 becomes conductive. As a result, the field current is increased to raise the power generation voltage of the generator 2.
Thus, the power generation voltage of the generator 2 is controlled to the predetermined constant voltage due to the repeated on-off control of the field current.
However, when an automotive generator is driven to operate, it becomes necessary to suppress the power generation voltage in accordance with the operating condition of the vehicle to reduce the engine load, or on the contrary to facilitate the power generation voltage so as to rapidly charge the battery. Therefore, it is necessary to make it possible to set the power generation voltage to three or more kinds of levels.
Accordingly, there has been proposed a system that can change the voltage adjusted by the control unit in accordance with a control signal from an external control unit.
Such a kind of control system for a vehicular generator is described, for example, in Japanese patent application laid-open No. S62-107643. In this case, however, there arises a problem that in order to set the power generation voltage at three levels, two external input terminals are required, resulting in an increased number of wirings for the control unit.
In addition, Japanese patent No. 3102981, for example, is given as a control system in which only a single input terminal from an external control unit is provided for arbitrarily adjusting a control voltage. In this case, however, it is necessary to arrange a circuit for determining an external input signal inside the control unit, so the construction of the control unit becomes very complicated. As a result, there is a problem that an increase in costs can not be avoided.